1. Field of the Invention
The present invention relates to an acoustic apparatus which measures acoustic characteristics in an acoustic space, a method of controlling the apparatus, and a program.
2. Description of the Related Art
The impulse response between a sound source and a sound receiving point in an acoustic space such as a room or hall includes important information concerning the acoustic characteristics of the space. For example, the user of an acoustic apparatus can obtain an acoustic effect as if he/she were listening to music in a given famous hall by storing the impulse response measured in the hall in the storage unit of the apparatus and performing filtering processing by applying the impulse response to a music signal to be reproduced. In some cases, the user may place a microphone at a listening point in a room where he/she listens to music, and reproduce a measurement sound signal from each speaker to measure the impulse response in that room between each speaker and the listening point.
This impulse response is used to generate a sound field correction filter which flattens the irregularity of the frequency response (“f characteristic”) of an impulse response which is generated by the interference between direct sound and reflected sound in a room, especially the peaks and dips of a low-frequency standing wave which exerts a considerable influence on audibility. In addition, it is possible to obtain a clear sound image by performing delay correction for the impulse responses between the respective speakers and the listening point so as to make the rise start times coincide with each other.
As described above, impulse responses are very useful to perform various kinds of acoustic processes in an acoustic apparatus. High importance is therefore attached to a technique of accurately measuring impulse responses, that is, a technique of suppressing the influence of noise on the measurement of impulse responses.
When measuring an impulse response in a room, the S/N ratio of a signal picked up by a microphone deteriorates because background noise always exists in the room. For this reason, Japanese Patent Laid-Open No. 2002-330500 uses a method of deciding the magnitude of a measurement signal by measuring the magnitude of background noise, and then securing a high S/N ratio relative to the background noise.
In order to obtain one impulse response, it is often a case where measurement sound signals are reproduced in a plurality of periods, and an arithmetic mean of the respective periodic signals in the picked-up signals is calculated, thereby reducing background noise and securing a high S/N ratio. Generally, the intervals at which measurement signals are reproduced are constant, and synchronous addition aims at reducing the random noise components of background noise. According to Japanese Patent Laid-Open No. 2007-232492, the intervals at which measurement sound signals are reproduced are changed at predetermined time intervals to reduce random noise by calculating arithmetic mean of the respective periods and, at the same time, inhibit the generation of frequencies that completely inhibit reductions in components other than random noise.
An actual space such as a room in which an acoustic apparatus is placed includes various noise sources such as an air conditioner, lights, a personal computer, and various kinds of electric appliances and devices. These noise sources generate noise from driving unit (for example, the hard disk, fan, and the like of the personal computer) where electric energy is converted into mechanical motion. For this reason, such noise basically has peak frequency components in narrow bands on the f characteristic. Background noise therefore includes these peak frequency components in addition to random noise components.
Recent acoustic apparatuses have been required to more frequently perform measurement of impulse responses necessary for the design of sound field correction filters as the number of channels of speakers increases and listening areas as correction targets enlarge. This has increased the possibility that various peak frequency components of background noise will mix in the signal picked up by a microphone in accordance with the respective measurement places and measurement timings. In this case, peak frequency components as narrow-band noise are weak in energy. For this reason, besides being accustomed to such sound, it is difficult for the user to perceive them as noise. This makes it difficult to take countermeasures such as eliminating a noise source in advance.
In addition, peak frequency components in a picked-up signal appear when electrical noise having peak frequency components directly mixes in constituent elements of the acoustic apparatus as well as when peak frequency components are picked up as reproduced by the above noise source. With an enlargement of a listening area, in particular, a microphone cable must be routed a long distance, and hence electrical noise picked up by the cable directly appears in a picked-up signal.
As described above, in practice, it is very likely that while peak frequency components as background noise will mix in picked-up signals actually acquired by an acoustic apparatus.
When an impulse response is calculated by using a picked-up signal including peak frequency components, since a high S/N ratio cannot be secured near the peak frequency components, a steep peak appears on the f characteristic of the impulse response obtained as shown in FIG. 3A. This peak is not based on the interaction between a speaker which reproduces music signals and the room, and hence should not be corrected by a sound field correction filter. However, when an acoustic apparatus automatically generates a sound field correction filter, it is impossible to discriminate noise as peak frequency components of background noise on the f characteristic. For this reason, for example, a notch filter having an extreme characteristic as indicated by the dotted line in FIG. 3B is generated as one of the constituent elements of a sound field correction filter.
Such a characteristic may affect the overall design of a sound field correction filter. Applying such a filter to the original characteristics based on the interaction between the speaker and the room shown in FIG. 3C will lead to a deterioration in sound quality such as the sound loss of music. That is, peak frequency components themselves are difficult to perceive by the user, but appear as a large peak on the f characteristic of a measured impulse response. For this reason, using an improper sound field correction filter to correct such noise will make the user perceive the peak as a deterioration in sound quality.
As described above, it is important to take countermeasures against peak frequency components of background noise, because they greatly affect the measurement of an impulse response and correction processing performed based on the measurement result.
The method disclosed in Japanese Patent Laid-Open No. 2002-330500, which increases a measurement signal in accordance with the magnitude of background noise, however, uniformly increases the S/N ratio for all the frequency bands. This makes it impossible to secure a satisfactory S/N ratio for peak frequency components of background noise. In addition, forcedly increasing a signal may cause clipping or distortion.
The method disclosed in Japanese Patent Laid-Open No. 2007-232492, which changes the sound reproduction intervals of measurement signals, is free from the problem that arbitrary peak frequency components other than random noise are not reduced at all, unlike the general arithmetic mean in which the sound reproduction intervals of measurement signals are constant. However, this processing does not measure background noise and is not specialized for a specific peak frequency. For this reason, large peak frequency components of background noise remain even after arithmetic mean operation.